An inverter is a form of converter of electrical energy. For example, the inverter can be arranged as part of a system for controlling the flow of power between a d.c. power supply and an a.c. power system. An inverter will typically comprise a bank of power switching devices, such as insulated gate bipolar transistors (IGBT's) and their anti-parallel diodes. The inverter is controlled by a microprocessor-based control unit which implements a control algorithm for the inverter. For example, power supplied from photovoltaic (PV) panels is fed to an a.c. power system such as an electrical grid network through an inverter that converts the d.c. output of the PV panels into a.c. power for the grid. In a particular application where the d.c. connections of an additional inverter supplying a motor are connected to create a drive that allows bidirectional power flow.
Inductors are arranged between the inverter phase outputs and the a.c. power system to limit the harmonic currents that would be created due to the switching operation of the inverter. An additional switching frequency filter is often present as well to limit the harmonic voltage distortion to an acceptable level at the point of connection with the a.c. power network.
Transient voltage changes can occur in an a.c. power system for various reasons, such as faults or the sudden connection of addition of equipment to the system. The inductors serve to limit the resulting transient current flowing between the a.c. power system and the inverter. An over-current protection system is usually provided to disable the switching devices in the inverter if the current flowing through the inverter exceeds a specific level above which the inverter may be damaged. A sufficiently large voltage transient in the a.c. power system may produce a current transient that is large enough to trigger the over-current protection system. However, disabling an inverter is seen as a last resort due to the time taken for the inverter to resynchronise itself to the supply and then undergo a restart procedure. It is not usually practicable to significantly increase the control bandwidth of the inverter control system to limit current transients so that the over-current trip is not activated with all possible voltage transients, and so the method conventionally used to ensure that the inverter remains active during a voltage transient in the a.c. supply is to increase the inductance between the a.c. supply and the inverter. In this way the transient current between the a.c. supply and the inverter can be limited, even with the worst case supply voltage transient, so that the over-current protection system is not activated except in the most severe of circumstances, such as a fault within the inverter system. Nevertheless, the impedance required to achieve this is larger than would be required for normal inverter operation. Large inductances are both bulky and expensive, and to be avoided if at all possible.